1. Field of the Invention
The invention relates to a liquid crystal display device and a method of fabricating the same, and more particularly to an in-plane switching active matrix type liquid crystal display device and a method of fabricating the same.
2. Description of the Related Art
For instance, Japanese Patent Application Publications Nos. 11-119237 and 10-186407 suggest in-plane switching active matrix type liquid crystal display devices in which a common electrode is designed to overlap a data line to shield an electric field generated at the data line in order to prevent the electric field from reaching a pixel for achieving a high numeral aperture and high accuracy.
However, the in-plane switching active matrix type liquid crystal display devices are accompanied with a problem of an increase in parasitic capacity between a common electrode and a data line. In order to reduce such parasitic capacity, there is suggested a colorless transparent film having a low dielectric constant, as an interlayer insulating film to be formed between a common electrode and a data line. For instance, such a colorless transparent film is comprised of a silicon nitride (SiNx) film as an inorganic film or an acrylic film as an organic film.
However, since a silicon nitride film is formed slowly by chemical vapor deposition (CVD), it would take much time to form a silicon nitride film having a thickness of 1 micrometer or greater. In addition, it is necessary to prepare a photolithography line including an expensive coating unit in order to form an acrylic organic film.
FIGS. 1 and 2 illustrate a conventional in-plane switching active matrix type liquid crystal display device. FIG. 1 is a plan view of a TFT substrate 100 on which a thin film transistor (TFT) is fabricated, viewed from liquid crystal, and FIG. 2 is a cross-sectional view taken along the line II—II in FIG. 1.
As illustrated in FIG. 2, the liquid crystal display device is comprised of a TFT substrate 100, a substrate 200 opposed to the TFT substrate 100, and a layer of liquid crystal 220 sandwiched between the TFT substrate 100 and the substrate 200.
The TFT substrate 100 is comprised of a first transparent substrate 101 composed of glass, a comb-shaped common electrode 127 formed on an upper surface of the first transparent substrate 101 (hereinafter, a surface of a substrate closer to the liquid crystal 220 is called an “upper” surface, and a surface of a substrate remoter from the liquid crystal 220 is called a “lower” surface), a gate line 105 (see FIG. 1) formed on an upper surface of the first transparent substrate 101, a first interlayer insulating inorganic film 106 formed on an upper surface of the first transparent substrate 101, covering the common electrode 127 therewith, a data line 112 formed on the first interlayer insulating inorganic film 106, a comb-shaped pixel electrode formed on the first interlayer insulating inorganic film 106, a second interlayer insulating inorganic film (passivation film) 115 formed on the first interlayer insulating inorganic film 106, covering the data line 112 and the pixel electrode 113 therewith, an alignment film 120 formed on the second interlayer insulating inorganic film 115, a polarizer 130 formed on a lower surface of the first transparent substrate 101, and a thin film transistor (not illustrated in FIG. 2).
The thin film transistor is comprised of an island 109 formed in the same layer as the common electrode 127, a drain electrode 110 and a source electrode 111 both formed in the same layer as the data line 112, and the above mentioned gate line 105.
The substrate 200 is comprised of a second transparent substrate 201 composed of glass, a black matrix layer 202 formed partially on an upper surface of the second transparent substrate 201, a color layer 203 formed partially on both an upper surface of the second transparent substrate 201 and the black matrix layer 202, a planarized layer 204 covering the black matrix layer 202 the color layer 203 therewith, an alignment film 120 formed on the planarized layer 204, an electrically conductive layer 205 formed on a lower surface of the second transparent substrate 201, and a polarizer 210 formed on the electrically conductive layer 205.
Spacers (not illustrated) are sandwiched between the TFT substrate 100 and the substrate 200 to provide a layer of the liquid crystal 220 with a constant thickness.
Seal (not illustrated) is sandwiched between the TFT substrate 100 and the substrate 200 at their marginal areas in order to prevent leakage of the liquid crystal 220.
In the liquid crystal display device illustrated in FIGS. 1 and 2, the common electrode 127 located beside the data line 112 was necessary to have a large area in order to prevent electric lines of force derived from the data line 112 from entering the pixel electrode 113. As a result, the liquid crystal display device illustrated in FIGS. 1 and 2 is accompanied with a problem that it is not possible to increase a numerical aperture.
In order to increase a numerical aperture, the common electrode 127 may be formed in a layer closer to the liquid crystal 220 than the data line 112 for shielding the data line 112 with the common electrode 127, in which case, an interlayer insulating organic film is formed between the data line 112 and the common electrode 127 for reducing a coupling capacity of the data line 112 and the common electrode 127.
An interlayer insulating organic film is formed generally by steps of coating photoresist, that is, liquid of organic resin fluidized by solvent and having photosensitivity, onto a film in a predetermined thickness by slit-coating or spin-coating by means of a coating unit, applying photolithography (exposure of the photoresist to light, development of the photoresist, and baking the photoresist) to the photoresist, and carrying out etching to the film with the photoresist being used as a mask.
An interlayer insulating organic film is composed usually of acrylic resin.
Acrylic resin has an advantage that a pixel in a liquid crystal display device may be composed of acrylic resin, because it is transparent, however, has disadvantages as follows.
First, it is impossible to use a coating unit through which novolak photoresist is coated onto a film, in a photolithography step, for coating acrylic resin together with novolak resin onto an object. Hence, it is necessary to prepare a coating unit used only for coating acrylic resin onto an object.
Second, since developing solutions used for acrylic photoresist and novolak photoresist are different from each other, it is not possible to develop acrylic photoresist together with novolak photoresist in a photolithography step by means of a developing unit used for developing novolak photoresist. Hence, it is necessary to prepare a developing unit used only for developing acrylic photoresist.
Third, it is not possible to store acrylic photoresist at room temperature. Hence, it is necessary to keep acrylic photoresist cool.
Fourth, acrylic photoresist tends to increase its viscosity with the lapse of time at room temperature.
Fifth, since acrylic photoresist is readily caked, it would be unavoidable to frequently carry out maintenance to a coating unit.
Sixth, acrylic photoresist is more expensive than novolak photoresist.
In contrast, novolak resin has only one disadvantage that since it is colored, it is, impossible to compose a pixel in a liquid crystal display device of novolak resin.